1. Field of the Invention
This invention relates to a transmitting and receiving system conducting a bi-directional communication between a transmitting and receiving means having an antenna coil and a transponder having a transponder coil electromagnetically coupled to the antenna coil.
2. Description of Related Art
In an automobile, to improve security against stealing, for example, a function such as an electronic key storing an electrical identification code therein is added to an ignition key of the automobile. On the automobile side, a transmitting and receiving Electrical Control Unit (ECU) has an immobilizer function that reads the identification code formed on the ignition key. While the ignition key is inserted in a key hole and manipulated to an ON position (or an Accessory (ACC) position) for engine start, the engine starts only when the code read from the ignition key coincides with the identification code that has been preset.
To realize such a transmitting and receiving system, the system is employed to conduct a bi-directional communication between the transmitting and receiving ECU and the transponder which is disposed on an ignition key side. More specifically, the transmitting and receiving system includes an antenna coil disposed around a key hole on a side of the transmitting and receiving ECU. The system transmits, through this antenna coil, a carrier signal that serves as a power source for the transponder, and a question signal that is modulated with this carrier signal.
The transponder placed on the ignition key side includes: a resonance circuit that has a transponder coil which is electromagnetically coupled to the antenna coil, and a resonance capacitor; a modulation circuit having a circuit element to change an impedance synthesized with the resonance circuit; a transmitting and receiving portion having a power source circuit for obtaining power by rectifying the carrier signal. The identification code stored on the ignition key side is transmitted to the transmitting and receiving ECU from the transponder as a response signal in response to the question signal. The transmitting and receiving portion of the transponder changes an impedance status in response to the question signal generated by a modulation circuit on the transmitting and receiving ECU side through the antenna coil.
Circuit constants of the transponder coil and the resonance capacitor are set to be values satisfying the resonance condition of the resonance circuit corresponding to the carrier signal frequency. The modulation circuit for changing the impedance of the transmitting and receiving portion on the transponder side generally has a serial circuit that includes a modulation capacitor and a switching element which is coupled in parallel to the resonance capacitor of the resonance circuit. The switching element is turned ON and OFF corresponding to the response signal to change the impedance of the transmitting and receiving portion.
However, in actual systems, the impedance change amount of the transmitting and receiving portion may not be ensured adequately due to deviations of circuit constants of components of the transmitting and receiving portion such as the transponder coil, the resonance capacitor, and the modulation capacitor, and due to shifts accompanied by circumstantial temperature changes of those circuit constants. If such an impaired state occurs, the transponder cannot return the response of the response signal, thereby causing a major problem that the transmitting and receiving system itself loses its function.